1. Field of the Invention
The present invention relates to an EMP generator. In particular, it relates to such a generator having at least one spark gap with two approximately spherical poles. It furthermore particularly relates to such a generator having at least two spark gaps connected in series with several capacitors and a power supply connected to the said series circuit via several resistors. The abbreviation EMP stands for "electro-magnetic pulse". The EMP generator is intended to be used for EMP simulation by means of current injection.
2. Discussion of Background
Standards committees, governmental authorities responsible for EMP and, above all, industry as equipment and plant suppliers are increasingly occupied with the search for realistic cost-effective EMP simulation methods.
In EMP simulation, it is a matter of investigating the effects of electro-magnetic interference fields, particularly of the so-called NEMP, on electrical or electronic systems, of taking suitable protective measures and checking these measures for their effectiveness. The NEMP is an electromagnetic pulse with a rise time of a few nanoseconds, a half-value period of decay of approximately 200 nanoseconds and a field strength of approximately 50 kV/m, to be expected when an atomic bomb is exploded outside the earth's atmosphere.
It is frequently too expensive and often not even possible, mainly in the case of relatively large equipment and particularly in the case of major installations, to expose the systems to be investigated directly to a field radiation.
The current injection technique previously mentioned offers an alternative here because the currents caused by the fields in the systems are, after all, the cause for their disturbance or destruction.
In the current injection technique, the system to be investigated is directly loaded with a current generated by a generator. Alternatively, a voltage possibly resulting in a current flow is applied to the system. As a rule, the systems to be investigated are electrical or electronic devices which are already provided with shielding. In these cases, the current is simply fed in via one of the plug-in connections present in most cases on the shielding housing. A standard regulation of US Navy MIL STD 461C prescribes for this type of test a generator which generates a current or voltage pulse of 10A or 1 kV, respectively, which oscillates at a frequency between 10 kHz and 100 MHz, across a resistive load. The standard is intended to ensure that the system test occurs under conditions as are expected in the case of an NEMP.
In fact, measurements in EMP field simulators have produced current or voltage shapes and intensities of the above type in electrical or electronic systems when irradiated with a pulse corresponding to the NEMP. However, the standardized test method exhibits grave disadvantages and not inconsiderable risks.
The main problem consists in the fact that the generators to be used in accordance with the standard generate the prescribed pulse only across a 100 k.OMEGA. resistance but that quite different pulse rates can be produced in connection with the system to be tested being the load. An adaptation of the generator to the respective load, that is to say to the respective characteristics of the system to be tested in each case is not provided for.
Neither are generators known in which such adaptation could be easily carried out. However, such an adaptation is absolutely necessary to obtain a test and investigation result which is meaningful even to some extent.
Another problem arises from injecting the test pulse directly into the system. In the case of shielded systems, electromagnetic fields primarily always cause currents to flow on the surface of the shielding. The voltages and currents occurring within the shielding are only the secondary consequence of these surface currents and are caused by coupling-in. The magnitude and the type of couplings depends on many factors. The typical shield attenuation is about 40 dB. As a rule, the pulses are not transferred with the same shape. The currents and voltages occurring inside the shielding therefore exhibit a variation which differs from the currents on the shieldings.
However, the effect of the currents and voltages coupled in on the system significantly depends on their variation with time. Thus, for example, the occurrence of flashovers or the responding of protective elements such as, for example, spark gaps, is affected by the steepness of the pulses. Naturally, the question whether and when a flash-over occurs or a protective element responds are decisive criteria for the behavior of a system under interference loading. By injecting the current or voltage pulse directly into the interior of the system to be investigated, all the effects occurring with the coupling-in from its shielding are neglected by the measuring technique.
In addition to an adaptation of the generator and the current or voltage pulses generated by it to the system to be investigated in each case, they should therefore not be applied in the interior of the system but on its shielding. A further important reason for this is that the surface currents occurring due to the electromagnetic interference fields on the shieldings can still be relatively well predicted by computation but their coup- ling-in behavior into the system cannot.
Naturally, applying the current or voltage pulses to the shielding has the consequence that current intensities or voltages which are higher by the magnitude of attenuation of approximately 40 dB must be generated and supplied by the generator.
Known generators which, in principle, meet the aforementioned requirements, are very large, unwieldy, virtually untransportable and expensive devices.
Another problem in the case of the known generators is that they supply either a current pulse or a voltage pulse. In the case of interference being coupled-in into a system, however, voltages and current frequently occur successively. First a voltage is built up. When this voltage exceeds a particular level (depending on the steepness of the voltage rise), for example, a flash-over occurs and a current flow follows. Thus, the system to be investigated automatically switches from a voltage pulse to a current pulse.
These and other objects are achieved according to the present invention by providing an EMP generator having at least one spark gap with two approximately spherical poles, wherein the spark gap is arranged in a housing having an internal space which can be placed under excess pressure compared with its environment via a first opening and a first compressed-air line connected thereto; a folded bellows projects into the internal space of the housing and is closed off with respect to this internal space; one of the poles of the spark gap is attached to the folded bellows; the internal space of the folded bellows is connected to a second compressed-air line; and the internal space of the folded bellows is capable of being placed via the second compressed-air line under an excess pressure which deviates from the aforementioned excess pressure in the internal space of the housing but is approximately of the same magnitude.